In order to understand trapped oil pressure and the different numbers of teeth effect on lubricating state on all tooth profile of external spur-gear high pressure oil pump, based on the geometrical relationship of gear transmission, with radius of curvature of driving gear in mesh-point engaged as a variable in a tooth period, all tooth profile was divided into three mating sections and eight especial points and seven courses，the corresponding transient formula of both oil film thickness based on elastohydrodynamic lubrication theory and tooth force were derived, hereby, the accurate position of minimum oil film thickness and maximum tooth force were estimated. Research and all results of an example indicated transient tooth force had a high change in a tooth period, the lubricating state on all tooth profile of external spur-gear high pressure oil pump is boundary lubrication of stiffness with a variable viscosity on the whole, different formula of oil film thickness should be used for different position of all tooth profile, different numbers of teeth have a positive unapparent effect on lubricating state on all tooth. Finally, a conclusion was educed that the gear transmission of external spur-gear high pressure oil pump was indeed different from conventional gear transmission in lubricating state on meshing surface, and the calculation of oil film thickness in external spur-gear pump couldn't directly use the conventional formula for conventional gear transmission.